Basic minimum clearances for messengers and insulated cables
are specified in Rules 37
and 38
, Tables 1
and 2
respectively and shall be measured to the nearest surfaces of the cable
and messenger assembly, including cable rings and messenger supports. The
basic clearances specified for conductors, as modified by provisions of
Rule 54.4
are applicable to insulated cables. These clearances for messengers and
cables are also subject to the following modifications. The clearances required
for supply conductors of 0 - 750 volts shall be applied to suitably insulated
cables of any voltage having a metallic sheath which is bonded and grounded
as specified in Rule 57.8
.

(1) Which Transport Freight Cars: The clearances specified in
Table 1, Case 1, Columns A, B, and D
are based upon the maximum height of standard freight cars, 15 feet 1 inch
from the top of rail to top of running board. These clearances shall in no
case be reduced more than 5% because of temperature, wind or mechanical loading.

(2) Operated by Overhead Trolley: The clearances specified in
Table 1, Case 2, Columns A and D
, 26 feet and 27 feet respectively, are based upon a maximum trolley pole
throw of 26 feet, the usual maximum height of a free trolley pole above the
top of rails or surface of streets used by trolley cars or coaches.

EXCEPTION: Messengers, and cables,
which are bonded and grounded as specified in
Rule 57.8
, may have clearances above the rails or surface of streets used by trolley
cars or coaches less than the distance specified in
Table 1, Case 2, Columns A and D
for trolley–throw clearance, provided the messengers and cables are not
less than 4 feet above the trolley contact conductor and are encased in
wood boxing or moulding with a minimum thickness of 7/8 inch (or other suitable
material having the same minimum electrical insulating qualities and mechanical
strength) for distances of at least 2 feet horizontally from the vertical
plane through each trolley contact conductor.

No specified clearance is required
between the individual conductors of suitably insulated cables, whether
single or grouped, and therefore the clearances specified in
Table 2, Cases 15, 16 and 17
do not apply between such conductors. Any such multiple–conductor group shall
be treated as a single conductor in the application of other clearance requirements.

D. Between
Cables and Messengers

Cables supported by messengers
are not required to be any specified distance from their supporting messengers.

E.
Between Messengers

The clearance specified
in Table 2, Case 1, Column
A
does not apply and no clearance is required where messengers of the same
electrical system branch or cross.

F.
From Poles, Crossarms and Other Conductors

Messengers, and
metal sheathed cables, which are bonded and grounded as specified in
Rule 57.8
shall have clearances not less than the minimum clearances required for conductors
of 0 - 750 volts with all of the following provisions being applicable:

(1) Attached to Surfaces of Poles and Crossarms: Such grounded messengers
and cables may be attached to the surfaces of poles or crossarms at less than
the clearance specified in Table
1, Column D, Cases 8 and 9
. When attached to pole or crossarm at less than 15 inches from centerline
of pole such grounded messenger or cable shall not be less than 4 feet vertically
above or 6 feet vertically below any unprotected supply conductor except
that where a guard arm (or arms) is placed above messenger and cable as specified
in Rule 57.7
, the clearance of 6 feet may be reduced to not less than 4 feet below unprotected
conductors of 0 - 750 volts (see
Figure 57–1 A
).

(2) Supported on Same Crossarm above 750 Volts: Such grounded messenger
and cable when supported on the same crossarm with unprotected conductors
in excess of 750 volts shall be placed on the side of pole opposite the unprotected
conductors (see
Figure 57–1 B
).

(3) Supported on Same Crossarm 0 - 750 Volts: Such grounded messenger and
cable when supported on the same crossarm with unprotected conductors of 0
- 750 volts, shall be placed at not less than pin spacing (
Table 2, Case 15, Column D
) beyond the outermost unprotected conductor on the same side of pole (see
Figure 57–1 C
).

(4) Between Cable and Unprotected Conductors: The clearances between
a cable and unprotected conductors shall not be held to apply between a grounded
cable (and its messenger) and unprotected conductors of the same circuit
on poles where unprotected conductors enter (or leave) a cable. On such poles
no grounded section of messenger or cable shall be less than 15 inches from
surface of pole.

(5) Two or More Cables Attached to the Surface of Pole: Where two or
more cables are attached to the surface of the same wood pole they shall
be placed on the same side of pole when their vertical separation is less
than 8 feet but in no case shall the vertical separation be less than 1 foot.

Messengers, and metal sheathed cables, which are bonded and grounded as
specified in Rule 57.8
are not required to clear buildings and other structures any specified horizontal
distance and the clearance specified in
Table 1, Case 7, Column D
does not apply in such instances. Such cables, and messengers, shall be
installed so that they do not interfere with the free use of fire escapes,
exits, etc., and should be arranged so as not to hamper or endanger workers
and firefighters while performing their duties.

Messengers and metal–sheathed cables which are bonded and grounded as specified
in Rule 57.8
, passing (unattached) nonclimbable street lighting and traffic signal poles
or standards, including mastarms, brackets, and lighting fixtures may have
a clearance less than the 36 inches specified in
Table 1, Case 10, Column D
, but shall have a clearance of not less than 15 inches, except when the
conductors are mechanically protected from abrasion by materials specified
in Rule 22.8
. Such mechanical protections shall extend not less than 15 inches in each
direction along the cable from centerline of pole, standard, attaching mastarm
or fixture, whether passing above, below or alongside. The conductors shall
be installed in such a manner so as not to interfere with light distribution
from lighting fixtures and shall not hamper workmen changing lamps or maintaining
equipment.